Chemical vapor deposition (CVD) is a commonly used means for depositing films on a substrate, such as a semiconductor wafer. Typically, CVD processes rely, at least in part, on thermal energy of the semiconductor to initiate the desired reactions. In some instances, however, it is desirable to reduce or minimize the temperature of a semiconductor wafer to protect the structures and materials that are already present.
One example of a material that can be damaged by excessive heat is a film of aluminum. If excessive heat is used after a layer of aluminum is already in place on a semiconductor wafer, the aluminum may diffuse into the adjoining materials, including the substrate itself. That diffusion can adversely affect the electrical performance of the aluminum and the adjoining layers, potentially leading to a defective semiconductor chip.
One approach at reducing the temperature, or thermal budget, necessary for high quality film deposition on semiconductor wafers is the use of plasma-enhanced CVD (PECVD). In that process, the inlet gases are excited by a radio frequency field that produces a plasma region containing free electrons, normal neutral gas molecules, ionized gas molecules, ionized portions of broken-up gas molecules, and free radicals. Although the additional energy imparted in PECVD processes does assist in the formation of thin films on the semiconductor substrates at lower temperatures, the PECVD processes do require the additional expense associated with purchasing and operating the plasma generating equipment. Furthermore, existing CVD equipment must often be extensively modified to accommodate the plasma generation or new equipment designed for PECVD must be purchased.
Another disadvantage of PECVD processes is that the additional energy present in the plasma region can result in ion implantation, energetic neutral embedment, sputtering and associated damage to the semiconductor wafer. In an attempt to address the problems caused by the additional energy in the plasma region, Markunas et al. described a process and equipment for remote plasma-enhanced CVD (RPECVD) in which the plasma region is located away from the substrate as described in U.S. Pat. No. 4,870,030. That process still relies, however, on plasma generation to reduce the thermal energy required for CVD. As a result, the additional costs associated with purchasing and operating plasma generating equipment still remain. Furthermore, the CVD chambers must be extensively modified to provide for remote plasma generation.
As a result, a need exists for a CVD process with a low thermal budget that can be used chemical vapor deposition of films on thermally-sensitive semiconductor wafers.